1. Field of the Invention
The present invention relates to the differentiation of integrated circuit chips formed in a same semiconductor wafer.
2. Discussion of the Related Art
To form integrated circuits, a large number of separate integrated circuit assemblies or chips are formed in a semiconductor wafer. From one chip to another, the circuits may be functionally identical (homogeneous) or may implement different functions (heterogeneous).
It may be desirable to differentiate circuits carrying out a same function (homogeneous). For this purpose, a chip identifier or secret quantity in the form of a binary code stored in a non-volatile memory is generally used. To make this memory inaccessible to a user, the code must be set upon manufacturing. A read-only memory is then used, formed in the same chip as the circuit thus identified. The corresponding memory cells comprise at least one MOS-type transistor. Upon circuit design, the gates of some memory cell transistors are eliminated to fix the state (on or off) of these transistors and thus program the concerned memory cells. A disadvantage is that this then modifies the physical aspect of the chip (the presence or the absence of the gates on the field insulation areas which separate the active areas of the transistors can be optically detected). The stored code can then be reconstructed based on this detection. Now, when a chip is desired to be provided with an identifier upon manufacturing, this often is because the secret of this identifier is desired to be kept.
The present invention aims at providing a method for differentiating functionally identical integrated circuits formed in a same semiconductor wafer, which overcomes the disadvantages of known methods.
The present invention more specifically aims at enabling storage of a binary code upon manufacturing, without for this code to be detectable by an optical observation of the chip.
To achieve these and other objects, the present invention provides a method for differentiating integrated circuits reproducing identical functions by storage of a binary code in a non-volatile storage element provided in each circuit, the method comprising providing, for each circuit of a same reticle, at least one selective implantation of dopants of its storage element which is different from the selective implantations of the dopants of the storage elements of the other circuits.
According to an embodiment of the present invention, a single selective dopant implantation is modified from one circuit to another within a same reticle.
According to an embodiment of the present invention, the reticle is formed by incorporating therein at least one geometry defining the storage elements, which is different from one circuit to another on the same reticle.
According to an embodiment of the present invention, the storage element is formed of elementary cells which each comprise at least one MOS transistor.
According to an embodiment of the present invention, the implantation of drain and source regions of the MOS transistors is modified from circuit to circuit.
According to an embodiment of the present invention, the implantation of the gates of the MOS transistors is modified from circuit to circuit.
According to an embodiment of the present invention, elementary cells of the storage element comprise resistors.
According to an embodiment of the present invention, the implantation of the resistors is modified from circuit to circuit.
The foregoing objects, features and advantages of the present invention will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings.